1. Field of the Invention
The present invention relates to a structure of a tyre for a motorcycle.
2. Description of the Related Art
In a motorcycle, a body is greatly inclined during cornering. A tyre for the motorcycle requires a cornering stability in addition to a straight running stability. Conventionally, the tyre for the motorcycle has been variously improved in order to meet these requirements. In order to enhance the straight running stability and cornering stability of the tyre for the motorcycle, a structure of a carcass constituting the framework of the tyre and a structure of a belt reinforcing the carcass particularly play an important part.
In general, a carcass ply of the tyre for a motorcycle has a (2−0) structure, a (1+1−0) structure or a (1−1−0) structure. The carcass ply is reinforced by a belt ply. A typical belt ply has a belt cord wound upon an outside of the carcass ply (an outside in a radial direction of the tyre) in a circumferential direction of the tyre. In the belt ply, a band-shaped sheet including the belt cord is wound spirally upon the outside of the carcass ply (see Japanese Laid-Open Patent Publication No. 5-246210).
In the belt ply having such a structure, a force for constrainting the carcass ply is great. Therefore, the amount of deformation of the tyre for the motorcycle is small during high speed straight running. As a result, the straight running stability of the tyre for the motorcycle is increased. In addition, the band-shaped sheet constituting the belt ply is wound at approximately 0 degree with respect to the circumferential direction of the tyre, and the belt cord is arranged in almost the circumferential direction of the tyre. Accordingly, the tyre has a high shock absorption during straight running and a ride comfort can also be enhanced.
On the other hand, the belt cord is arranged at approximately 0 degree (which is equal to or smaller than 5 degrees) with respect to the circumferential direction of the tyre. Therefore, the tyre for the motorcycle has a small side force which is generated during cornering. For this reason, a high speed cornering performance tends to be deteriorated. In order to solve this problem, a cut ply is provided on the inside or outside of the belt ply in some cases. Consequently, a side stiffness of the tyre can be enhanced and the high speed cornering performance of the tyre can be improved.
In the case in which the cut ply is provided on the inside or outside of the belt ply, however, the stiffness of a tread portion of the tyre is greatly increased. For this reason, there is a problem in that the ride comfort is deteriorated and a weight of the tyre is increased.